1. Field of the Invention
The present invention relates generally to optical fiber, and more particularly to dispersion compensation fiber and transmission lines including combinations of transmission fiber and dispersion compensation fiber.
2. Technical Background
Higher data rates and wider bandwidth systems are becoming needed for the telecommunications industry. Thus, the search for high performance optical fibers designed for long distance, high bit rate telecommunications that can operate over broad bandwidths has intensified. These high data rates and broad bandwidths, however, have penalties associated with them. In particular, dispersion is a significant problem for such systems. More specifically, positive dispersion builds as a function of the length of the high data rate transmission fiber. Dispersion Compensating (DC) fibers included in cable or in Dispersion Compensation Modules (DCM""s) have been designed that compensate for such dispersion. These fibers generally have negative dispersion slope and negative total dispersion, with the dispersion having a large negative value such that a short length of the DC fiber compensates for the positive dispersion and positive slope of the longer transmission portion. For C- and L-band operation between 1525 nm and 1625 nm, the bend performance (both macro-bending and micro-bending) and other properties, such as dispersion and kappa linearity (kappa being the ratio of total dispersion divided by dispersion slope at a specific wavelength) of the DC fiber are very important. This is particularly true in DC fibers that will be included in a wound spool of a DCM, but also for cabled DC fiber utilized in dispersion managed systems.
Thus, there is a need for a DC fiber which: (1) exhibits fairly linear properties over the C- and L-bands in a wavelength range (1525 nm to 1625 nm); (2) retains the usual high performance optical fiber characteristics such as high strength, low attenuation and acceptable resistance to micro- and macro-bend induced loss, and (3) is particularly effective at compensation for the dispersion of low slope transmission fibers across the C, L and C+L-bands with low average residual dispersion.
The following definitions are used herein.
Refractive Index Profilexe2x80x94The refractive index profile is the relationship between refractive index and optical fiber radius for the DC fiber.
Segmented Corexe2x80x94A segmented core is one that has multiple segments in the core, such as a first and a second segment (a central core and a moat, for example). Each core segment has a respective refractive index profile and maximum and minimum refractive index therein.
Radiixe2x80x94The radii of the segments of the core 21 are defined in terms of the beginning and end points of the segments of the refractive index profile of the fiber 20. FIG. 3 illustrates the definitions of radii R1, R2, and R3 used herein. The same dimension conventions apply for defining the radii in FIGS. 4-15, 28-30 and 43 as well. The radius R1 of the central core 22 is the length that extends from the DC fiber""s centerline CL to the point at which the refractive index profile crosses the relative refractive index zero 23 as measured relative to the cladding 28. The outer radius R2 of the moat segment 24 extends from the centerline CL to the radius point at which the outer edge of the moat crosses the refractive index zero 23, as measured relative to the cladding 28. The radius R3 is measured to the radius point at which a tangent to the outer edge 27 of the ring 26 meets the refractive index zero 23, as measured relative to the cladding 28. The width of the ring 26 is defined as the distance from R3 to the bisecting point of a tangent of the inward portion 29 of the ring with the refractive index zero 23, as measured relative to the cladding 28.
Effective Areaxe2x80x94The effective area is defined as:
Aeff=2xcfx80(∫E2r dr)2/(∫E4r dr),
where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light as measured at 1550 nm.
xcex94% or Delta (%)xe2x80x94The term, xcex94% or Delta (%), represents a relative measure of refractive index defined by the equation,
xcex94%=100(ni2xe2x88x92nc2)/2ni2
where ni is the maximum refractive index (highest positive or lowest negative) in the respective region i (e.g., 22, 24, 26), unless otherwise specified, and nc is the refractive index of the cladding (e.g., 28) unless otherwise specified.
xcex1-profilexe2x80x94The term alpha profile, xcex1-profile refers to a refractive index profile, expressed in terms of xcex94(b)%, where b is radius, which follows the equation,
xcex94(b)%=[xcex94(bo)(1xe2x88x92[|bxe2x88x92bo|/(blxe2x88x92bo)]xcex1)]100
where bo is the maximum point of the profile and bl is the point at which xcex94(b)% is zero and b is in the range bixe2x89xa6bxe2x89xa6bf, where xcex94% is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number. The initial and final points of the xcex1-profile are selected and entered into the computer model. As used herein, if an xcex1-profile is preceded by a step index profile, the beginning point of the xcex1-profile is the intersection of the xcex1-profile and the step profile. In the model, in order to bring about a smooth joining of the xcex1-profile with the profile of the adjacent profile segment, the equation is rewritten as;
xcex94(b)%=[xcex94(ba)+[xcex94(bo)xe2x88x92xcex94(ba)]{(1xe2x88x92[|bxe2x88x92bo|/(blxe2x88x92bo)]xcex1}]100,
where ba is the first point of the adjacent segment.
Pin array macro-bending testxe2x80x94This test is used to compare relative resistance of optical fibers to macro-bending. To perform this test, attenuation loss is measured when the optical fiber is arranged such that no induced bending loss occurs. This optical fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements in dB. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center-to-center. The pin diameter is 0.67 mm. The optical fiber is caused to pass on opposite sides of adjacent pins. During testing, the optical fiber is placed under a tension sufficient to make the optical fiber conform to a portion of the periphery of the pins.
Lateral load testxe2x80x94Another bend test referenced herein is the lateral load test that provides a measure of the micro-bending resistance of the optical fiber. In this test, a prescribed length of optical fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. A known length of optical fiber is sandwiched between the plates and the reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation in dB/m is measured. This increase in attenuation is the lateral load attenuation of the optical fiber.
In accordance with embodiments of the present invention, a dispersion compensation fiber is provided having a refractive index profile including a core having a central core with a core delta (xcex941) having a value greater than 1.5%, a moat surrounding the central core having a moat delta (xcex942) having a value less negative than xe2x88x920.65%, and a ring surrounding the moat having a positive ring delta (xcex943). The refractive index profile of the DC fiber is selected to provide a total dispersion less than xe2x88x9287 and greater than xe2x88x92167 ps/nm/km at 1550 nm; a dispersion slope more negative than xe2x88x920.30 ps/nm2/km at 1550 nm; and a kappa value defined as the total dispersion at 1550 nm divided by the dispersion slope at 1550 nm of greater than 151 and less than 244 nm, and a core-moat ratio, defined as a radius (R1) to the outer edge of the central core divided by a radius (R2) to the outer edge of the moat, of greater than 0.31.
In accordance with other embodiments of the invention, a dispersion compensation fiber is provided comprising a refractive index profile having a core including a central core with a core delta (xcex941) of less than 2.2%, a moat surrounding the central core having a moat delta (xcex942) less negative than xe2x88x920.65%, and a ring surrounding the moat having a positive ring delta (xcex943). The refractive index profile of the DC fiber is selected to provide a total dispersion less than xe2x88x9296 and greater than xe2x88x92130 ps/nm/km at 1550 nm; a dispersion slope more negative than xe2x88x920.35 and less negative than xe2x88x920.85 ps/nm2/km at 1550 nm; and a kappa of greater than 163 and less than 219 nm.
In another embodiment of the invention, an optical transmission line is provided. The transmission line includes a transmission fiber with a total dispersion between 4 and 10 ps/nm/km at a wavelength of 1550 nm, and a positive dispersion slope of less than 0.038 ps/nm2/km at a wavelength of 1550 nm, and a dispersion compensation fiber optically connected to the transmission fiber, the dispersion compensation fiber having a refractive index profile including a core with a central core having a core delta (xcex941) having a value greater than 1.5%, a moat surrounding the central core having a moat delta (xcex942) having a value less negative than xe2x88x920.65%, and a ring surrounding the moat having a positive ring delta (xcex943), the refractive index profile of the dispersion compensation fiber selected to provide a total dispersion less than xe2x88x9287 and greater than xe2x88x92167 ps/nm/km at 1550 nm, a dispersion slope more negative than xe2x88x920.30 ps/nm2/km at 1550 nm; kappa of greater than 151 and less than 244 nm, and a core-moat ratio, defined as a radius (R1) to the outer edge of the central core divided by a radius (R2) to the outer edge of the moat, of greater than 0.31.
In accordance with another embodiment, an optical transmission line is provided comprising a transmission fiber having a total dispersion between 4 and 10 ps/nm/km at a wavelength of 1550 nm, and a positive dispersion slope of less than 0.045 ps/nm2/km at a wavelength of 1550 nm, a dispersion compensation fiber having a length between 4 and 8 km optically coupled to the transmission fiber including a refractive index profile with a core having a central core having a core delta (xcex941) of less than 2.2%, a moat surrounding the central core having a moat delta (xcex942) less negative than xe2x88x920.65%, and a ring surrounding the moat having a positive ring delta (xcex943), the refractive index profile of the dispersion compensation fiber selected to provide total dispersion less than xe2x88x9296 and greater than xe2x88x92130 ps/nm/km at 1550 nm; dispersion slope more negative than xe2x88x920.35 and less negative than xe2x88x920.85 ps/nm2/km at 1550 nm; and kappa, defined as the total dispersion at 1550 nm divided by the dispersion slope at 1550 nm, of greater than 163 and less than 219 nm, and a Raman pump optically coupled to the dispersion compensating fiber.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.